In digital communications systems, a carrier signal is modulated with the digital data to be transmitted over the channel, where it typically suffers various forms of distortion, such as additive noise. The digital data is often transmitted in bursts wherein each burst consists of a number of data bits. Upon reception, the signal must be demodulated in order to recover the transmitted data.
It is common for receivers to employ direct conversion (i.e. homodyne receiver) to perform the demodulation of the received signal. The received signal is mixed with a local oscillator signal at the carrier frequency to produce I (in-phase) and Q (quadrature) baseband signals. An advantage of direct conversion receivers is that they are efficient in terms of cost and current consumption. The advantage is derived from having the incoming RF signal directly downconverted to baseband, in both I and Q components, without use of any IF frequencies.
In other receivers, the incoming RF signal is mixed down first to an intermediate frequency (IF) signal. The IF frequency may be any frequency for which the implementation of the necessary amplification and/or filtering is convenient.
For example, in the Bluetooth receiver utilizing the present invention, the front-end outputs a low frequency IF signal that can cover a large range of amplitudes that can be represented by an 1-bit word after quantization. A problem arises, however, in that the IF signal may ride on a wide range of DC levels that is often significantly wider than the amplitude of the signal itself. For example, the DC level may be tens of dB larger when low level RF signals are received.
DC offsets in a receiver are typically introduced in the mixer in the front end portion of the receiver, but may also result from nonlinearities and mismatches in other circuits of the receiver. Leakage of the local oscillator signal and self-downconverting to DC through the mixer causes the DC components to be generated at a wide range of levels depending on various factors. In order to properly detect and decode the received signal, the DC components must be removed or suppressed. Since it is usually not practical to predict the exact DC offset and compensate for it without actually measuring it, a compensation mechanism is often needed which determines the level of undesired DC that must be eliminated from the signal. In the case of a Bluetooth receiver, the DC components must be removed before the IF to Zero-IF conversion (i.e. the second frequency conversion in the receiver). Additional causes of DC offsets being generated include transistor mismatch in the signal path, the presence of a large near-channel interferer leaking into the local oscillator and self-downconverting to DC at the mixer. These would also have a measurable effect on the signal at this point of the circuit and would therefore be compensated for by a mechanism that could determine the DC level at that point.
Digital demodulators used in the receiver are sensitive to the DC offsets. They typically require suppression of DC offsets such that the remaining residue is limited to 5% of the signal's amplitude. At this point, the performance degradation caused by the DC bias is tolerable (e.g., fractions of dB of degradation in BER versus Eb/No performance). For relatively weak signals, it is more difficult to eliminate the DC bias of the received signal with 5% relative accuracy (referenced to the signal's amplitude) due to the limited resolution of the quantizer.
An additional problem arises considering the high complexity multi-coefficient filters used in down conversion and image rejection stages in the digital receiver. Consequently, the input signal must be approximately adjusted to within +/−6 dB dynamic range in order to avoid performance degradation caused by either truncation or saturation during subsequent digital processing. In addition, it is desirable that any implementation of a solution to this problem has minimum gate count to reduce size, cost and current consumption. Therefore, there is a need for a mechanism that overcomes the problems associated with the prior art that is able to estimate the DC offset in a signal and compensate an input signal for the DC offset estimate.